c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my $data = [
   [ 123 ], [ 789 ], [ 'dup' ], [ 456 ], [ 'dup' ], [ 123 ], [ 543 ],
   ];
 dd $data;
 ;;
 my %seen;
 my $lo = 0;
 for (my $hi = 0;  $hi <= $#$data; ) {
   ++$seen{ $data->[$lo][0] = $data->[$hi][0] };
   ++$lo;
   ++$hi while $hi <= $#$data && $seen{ $data->[$hi][0] };
   }
 $#$data = $lo-1;
 dd $data;
"
[[123], [789], ["dup"], [456], ["dup"], [123], [543]]

[[123], [789], ["dup"], [456], [543]]